# Antennas & antenna coils for EM theremin

Posted: 7/8/2012 8:11:50 AM

From: Minsk, Belarus

Joined: 6/21/2012

Thierry:...a resonant frequency of about 259.6kHz which is well in the acceptable (but not optimal) range for an EM-Theremin...

What exactly do You mean?

Posted: 7/8/2012 12:34:02 PM

From: Colmar, France

Joined: 12/31/2007

What do I exactly mean?

Since the antenna circuit cannot be tuned, one has to follow an inverse procedure:

In order to get optimal linearity and pitch range, one should disconnect the first linearization coil from the variable oscillator, measure the resonant frequency of the antenna circuit alone (in the final environment, that means with the circuit board, antenna fixings, the whole instrument set up on a mic stand, no player beneath). Then tune the variable oscillator with a frequency counter to about the same frequency, still with no antenna circuit connected.

Now, connect the antenna circuit. The oscillator's frequency should go up by about 7kHz. Then tune the fixed pitch oscillator to this value and you have an optimal result.

When working with the default values (pitch oscillators at around 285kHz or 278kHz without antenna circuit) the resonant frequency of the antenna circuit will be ways too low which will limit the pitch range and worsen the linearity. That's why Bob Moog decided in the final release of the EM which was then called Etherwave to reduce linearization inductance to 30mH and to make the antenna circuit tunable by bending the wire which leads to the antenna above a grounded aluminum foil (thus adding some static capacitance).

Posted: 7/8/2012 12:36:54 PM

From: Colmar, France

Joined: 12/31/2007

For a deeper explanation see my 3 consecutive posts on this page.

Posted: 7/10/2012 2:37:33 PM

From: Theremin Motherland

Joined: 11/13/2005

antman>> Hey Fred, what does that k actually mean and what would it be if it wasn't mounted at almost ground level.  Is there a separate equation for k that you can point me to.

Thierry>> This k=0.4 is a "wet thumb" estimation which shall compensate the difference between theory and practice. The authors of the paper where it appeared for the fist time didn't give an exact reason for using just this value...

Parameter k is defined by the ratio h/L(corrected!) where L is a length af antenna and h is a distance between  antenna and conductive surface in the ideal model.

Some values of k:

h/L        k=

0.02 - 0.928

0.04 - 0.884

0.08 - 0.82

0.2 - 0.702

0.4 - 0.604

0.8 -  0.504

2 - 0.408

5 - 0.352

10 - 0.332

inf. - 0.307

Posted: 7/10/2012 3:28:16 PM

From: Eastleigh, Hampshire, U.K. ................................... Fred Mundell. ................................... Electronics Engineer. (Primarily Analogue) .. CV Synths 1974-1980 .. Theremin developer 2007 to present .. soon to be Developing / Trading as WaveCrafter.com . ...................................

Joined: 12/7/2007

"Parameter k is defined by the ratio L/h where L is a length af antenna and h is a distance between  antenna and conductive surface in the ideal model." - ILYA

Hi Ilya - sorry, but I am a bit thick! ;-)

"distance between  antenna and conductive surface in the ideal model." - What exactly does this mean? Distance from the bottom of a vertical antenna to "ideal" ground?

I suspect it must be "Distance from the bottom of a vertical antenna to ground" - And that, when applied to an equasion which accounts for antenna diameter, it all works - Hopefully ;-).

A infinite ratio bends my brain a bit, LOL - I think that in order to achieve this the antenna would pass through earth, and so be well and truly grounded! (or be a singularity... ?  ;-)

Fred.

Posted: 7/10/2012 4:15:24 PM

From: Theremin Motherland

Joined: 11/13/2005

Fred, I appreciate your British humor and would like to hear something, say, about STR.

Fixed.

>> Distance from the bottom of a vertical antenna to "ideal" ground?

Distance from the bottom of a vertical cylindrical conductor to infinite horizontal conductive plate if you like.

Posted: 7/11/2012 8:50:17 AM

From: Eastleigh, Hampshire, U.K. ................................... Fred Mundell. ................................... Electronics Engineer. (Primarily Analogue) .. CV Synths 1974-1980 .. Theremin developer 2007 to present .. soon to be Developing / Trading as WaveCrafter.com . ...................................

Joined: 12/7/2007

"Fred, I appreciate your British humor and would like to hear something, say, about STR." - Ilya

LOL ! - not so sure its "British humor" ;-) - I often use "humor" when I have absolutely no idea what i'm talking about .. as for "STR" - what the hell is that?

IMO, most "British humor" these days is crap! - It has degenerated to infant school level.. Severe decline since the days of Monty Python, Spitting Image and the like.

Anyway, I am not really sure that I identify myself with any nationality - I am something of a mongrel, having Russian grandfather, Scottish father, Dutch mother, and growing up in South Africa. I find myself most comfortable in the company of people from Russia and Eastern Europe (Poland, Czechoslovakia) and the Caribbean, and probably least comfortable in the company of South Africans and the English!

The cleverest humor I ever encountered was from a German engineer I worked with in the 70's - We travelled across South Africa together installing medical equipment, it took me a long time to understand his humor - but when I did I realised that almost everything he said was 'twisted' and had an alternate hillarious interpretation, And I realised this was deliberate!

Fred.

Posted: 7/11/2012 4:03:30 PM

From: Theremin Motherland

Joined: 11/13/2005

If you decide to play the theremin to the people of Alpha Centauri...  and if you go with the speed of light... how will you correct the reducing note spacing of your theremin (in accordance of Einstein's phenomenon) to be able rehearsing on the board of spaceship? (sorry, but the Special theory of relativity was my first association on a word "singularity")

Posted: 7/11/2012 8:41:00 PM

From: Eastleigh, Hampshire, U.K. ................................... Fred Mundell. ................................... Electronics Engineer. (Primarily Analogue) .. CV Synths 1974-1980 .. Theremin developer 2007 to present .. soon to be Developing / Trading as WaveCrafter.com . ...................................

Joined: 12/7/2007

If you decide to play the theremin to the people of Alpha Centauri...

No point - They havent evolved ears.

and if you go with the speed of light...

This is infinitely unlikely - therefore you could just use an infinite improbability drive to get you there at some improbable time.

how will you correct the reducing note spacing of your theremin (in accordance of Einstein's phenomenon) to be able rehearsing on the board of spaceship?

No need to rehearse - they are the ideal theremin audience, they're deaf!

Posted: 7/11/2012 11:35:39 PM

Joined: 7/5/2012

"Parameter k is defined by the ratio h/L(corrected!) where L is a length af antenna and h is a distance between  antenna and conductive surface in the ideal model." - ilya

Thanks so much for this, I will experiment with this and see what comes up.

Also, does anyone know if there's anything to gain using a frequency higher than 260 kHz?